The invention relates to a switching module comprising at least one junction switch for connecting a first bus to a second bus to transmit packetized digital information having an address field and a data field.
A junction switch in such a switching module is used for switching through packetized digital information in telecommunication networks. An example in this context is a network in which two or a plurality of computers are coupled for exchanging digital information.
Different structures are known for the network configuration, such as for example the Banyan network, Prelude network, Orwell rings and the Time-division mutiplex bus. Each of the said network structures will presently be briefly discussed.
(a) Banyan network.
This network is known from an article entitled "Performance Analysis of a Packet Switch Based on Single-Buffered Banyan Network" from IEEE Journal on Selected Areas in Communications; VOL SAC-1, No. 6; Dec. 1983, pp. 1014-1021.
The Banyan network consists of a matrix of modules each having two inputs and two outputs. The value of an address bit from the address field of a packet determines the output of the module to which the packet is switched through. The output is connected to a next module which switches the packet through in the same manner depending on the value of the next address bit. In this way n module stages are required for an address field of n address biis. Thus, a property of the Banyan network is the fact that the size of the network is determined by the size of the address field and that the size of the network cannot be designed in dependence on the traffic need
Furthermore, it is disadvantageous that when at any instant an equivalent address bit appears at the two inputs of a module, without further measures being taken, the information on that module is blocked, disturbing the throughput. To solve such problems, an intricate sorting mechanism would be required for sorting and distributing packets over the proper inputs.
(b) Prelude network
The Prelude network is known from the report of a conference of the GSLB, held in Albufeira, Portugal, 19-20 Jan. 1987; pp. 114-127.
A fixed number of ingoing and outgoing lines is connected to the Prelude network. The signals on the ingoing lines are presiochronous, that is to say, that the signals are nominally synchronous but no longer entirely synchronous with respect to each other owing to, for example, jitter and noise. In order to attain that the input signals are bit and word synchronous, a synchronization circuit is inserted between the ingoing lines and the rest of the switching network
Furthermore, additional circuitry is present for shifting the packets with respect to each other on the ingoing lines over a time interval having the length of an address field. Consequently, the address fields can be transmitted to a control unit whilst being equally distributed in time. Subsequently, the packets are further switched through the rest of the switching network.
A disadvantageous property of the Prelude network is the fact that the number of ingoing lines is limited (that is to say, equal to the ratio of the number of data bits to the number of address bits); furthermore, the ingoing lines must not function asynchronously with respect to each other and much circuitry is required for synchronizing the ingoing lines and shifting the packets with respect to each other.
(c) Orwell rings.
This network is known from the report of a conference by the GSLB, held in AIbufeira, Portugal; 19-20 Jan. 1987, pp.215-224.
The network comprises stations that are included in a ring-shaped bus This ring-shaped bus comprises one or a plurality of parallel rings. Frames containing digital information are transmitted on each ring.
This network has the disadvantageous property that the stations have to be synchronous with respect to each other; furthermore, the structure of the network cannot be simply changed into a structure different from the ring-shaped one and, in addition, the user capacity per station decreases as more stations are included in the ring, because there is a limitation to the used capacity of the bus. These properties form a restriction on the possibilities to use that network.
(d) Time-division multiplex bus.
Time-division multiplex buses are widely known. A number of ingoing data lines is connected via a multiplexer to a common time-division multiplex bus. Each ingoing line is read out during an associated time slot and the thus obtained data packet is placed onto the common bus during that time slot. Since the time slots belong to a fixed multiplex frame, the ingoing lines must not be asynchronous.
Each outgoing line that is connected to the common bus has a unique address. The outgoing lines jointly read the common bus and if a packet address field corresponds with an address of an outgoing line the packet is copied onto the relevant outgoing line.
In addition to the disadvantage that the ingoing lines must not be asynchronous, also high-speed buses are required when many lines are connected.
A known routing method with telecommunication networks is the use of virtual circuits. Virtual circuits are applied when a packet for a specific destination should be provided with a very long world address. To avoid this long world address, it is transmitted to a central control unit by a station wishing to commence transmission of the packets, which control unit establishes virtual circuits in response thereto. These virtual circuits are formed by issuing a Virtual Circuit Identifier, VCI for short, per section of the overall connection as well as the required routing information for these VCI values to the junctions in that section of the connection. These (much shorter) VCI values substitute for the world address. The central control unit informs each junction about the value the VCI of the arriving packet will have and which new value the VCI of that packet is to assume to route the packet to the next junction.
Each junction has a conversion table with a combination of two VCl values for each virtual circuit; one VCl associated to the arriving packet and one VCI with which that packet is to be routed further. When a packet arrives at a junction, the VCI of that arriving packet is looked up in the conversion table and replaced by the VCl that is associated to it according to the conversion table. Subsequently, the packet with the new VCI is routed to the next junction The VCl values associated to a virtual circuit cannot be used for other circuits as long as the virtual circuit is still in use Only after the central control unit has been informed that the communication can be terminated, the VCl values of the associated virtual circuits can be released by the central control unit.